In natural mineral oil deposits, mineral oil is present in the cavities of porous reservoir rocks which are sealed toward the surface of the earth by impermeable top layers. The cavities may be very fine cavities, capillaries, pores or the like. Fine pore necks may, for example, have a diameter of only approx. 1 μm. As well as mineral oil, including fractions of natural gas, a deposit comprises water with a greater or lesser salt content.
In mineral oil production, a distinction is drawn between primary, secondary and tertiary production.
In primary production, after commencement of drilling of the deposit, the mineral oil flows of its own accord through the borehole to the surface owing to the autogenous pressure of the deposit. According to the deposit type, however, usually only approx. 5 to 10% of the amount of mineral oil present in the deposit can be produced by means of primary production; thereafter, the autogenous pressure is no longer sufficient for production.
After primary production, secondary production is therefore used. In secondary production, in addition to the boreholes which serve for the production of the mineral oil, known as the production boreholes, further boreholes are drilled into the mineral oil-bearing formation. These are known as injection boreholes, through which water and/or steam is injected into the deposit, in order to maintain the pressure or to increase it again. As a result of the injection of the water, the mineral oil is gradually forced through the cavities in the formation, proceeding from the injection borehole, in the direction of the production borehole. However, this works only for as long as the cavities are completely filled with oil and the more viscous oil is pushed onward by the water. As soon as the mobile water breaks through cavities, it flows on the path of least resistance from this time onward, i.e. through the channel formed between the injection boreholes and the production boreholes, and no longer pushes the oil onward. By means of primary and secondary production, generally only approx. 30 to 35% of the amount of mineral oil present in the deposit can be produced.
It is known that the mineral oil yield can be enhanced further by measures of tertiary oil production. Tertiary oil production includes processes in which suitable chemicals are used as assistants for oil production. This includes what is called “polymer flooding”. Polymer flooding involves injecting an aqueous solution of a thickening polymer into the mineral oil deposit through the injection boreholes instead of water. As a result of the injection of the polymer solution, the mineral oil is forced through said cavities in the formation from the injection borehole proceeding in the direction of the production borehole, and the mineral oil is finally produced through the production borehole. Due to the elevated viscosity of the polymer solution, which is matched to the viscosity of the mineral oil, the polymer solution is thus able to break through cavities at least not as easily as is the case for pure water, if at all. Parts of the deposit not accessible to the water are reached by the polymer solution.
For polymer flooding, a multitude of different thickening water-soluble polymers have been proposed, both synthetic polymers, for example polyacrylamide or copolymers of acrylamide and other monomers, especially monomers having sulfo groups, and polymers of natural origin, for example glucosylglucans, xanthans or diutans.
Glucosylglucans are branched homopolysaccharides formed from glucose units. Homopolysaccharides formed from glucose units are called glucans. The branched homopolysaccharides mentioned have a main chain formed from β-1,3-bonded glucose units, of which—viewed statistically—about every third unit is β-1,6-glycosidically bonded to a further glucose unit. Glucosylglucans are secreted by various fungal strains, for example by the filamentous basidiomycete Schizophyllum commune, which, during growth, secretes a homopolysaccharide of the structure mentioned with a typical molecular weight Mw of approx. 5 to approx. 25*106 g/mol (trivial name: schizophyllan). Mention should also be made of homopolysaccharides of the structure mentioned secreted by Sclerotium rolfsil (trivial name: scleroglucans).
The production of such glucosylglucans is disclosed, for example, in EP 271 907 A2, EP 504 673 A1, DE 40 12 238 A1 and WO 03/016545, and they are produced specifically by fermenting suitable fungal strains while stirring and venting, and removing the polysaccharide formed.
Our prior application EP 09179716.7 discloses a process for producing concentrated glucosylglucan solutions with concentrations of more than 3 g/l.
CA 832 277 A discloses the use of aqueous solutions of glucosylglucans for polymer flooding. The aqueous solutions used have, at a concentration of 1% by weight, a viscosity at 24° C. of at least 500 mPa*s, and the concentration of the glucosylglucans is 0.005 to 1% by weight, preferably 0.01 to 0.3% by weight. The solutions used may additionally comprise further components, for example surfactants, biocides or bases, for example alkali metal hydroxides.
EP 271 907 A1 discloses a process for producing glucosylglucans, fungal strains particularly suitable for this purpose, and the use of such glucosylglucans for tertiary mineral oil production. The document further discloses measurements of the viscosity of aqueous solutions in saline water at temperatures of 25° C. to 60° C.
Udo Rau, Andreas Haarstrick and Fritz Wagner, Chem. Ing. Tech. 64(6) (1992), pages 576/577 propose the use of schizophyllan solutions for polymer flooding of mineral oil deposits with high temperature and salinity, without describing details of a process.
Udo Rau, “Biosynthese, Produktion and Eigenschaften von extrazellulären Pilz-Glucanen” [Biosynthesis, production and properties of extracellular fungal glucans] in Berichte aus der Biotechnologie, Shaker Verlag, Aachen, 1997, pages 106 ff mentions that schizophyllan solutions have thermal stability up to 135° C. and should therefore be suitable for tertiary mineral oil production in deep deposits, for example in the North Sea. It is also mentioned that schizophyllan has a reversible decrease in viscosity up to shear rate of 40 000 s−1. It is additionally pointed out that the viscosity of schizophyllan solutions is barely influenced by the presence of alkali metal and alkaline earth metal ions.